1. Field of the Invention
The present invention relates to a semiconductor device. More particularly, the present invention relates to a semiconductor device with solder balls as external connection terminals having high reliability.
2. Description of the Related Art
Conventionally, Sn—Pb eutectic solder has been used. However, since a lead-free solder is requested, various solders are developed.
For example, Japanese Laid-Open Patent Application JP-A-Heisei, 5-50286 discloses one of such solders. The solder (hereafter, referred to as Sn—Ag—Cu-based Solder) is composed of: Ag between 3.0 and 5.0 weight %; Cu between 0.5 and 3.0 weight %; and Sn of the remainder. The solder ball of such composition is actually used as the external connection terminal of BGA (Ball Grid Array).
FIG. 1 is a sectional view showing a conventional semiconductor device. As shown in FIG. 1, in the conventional semiconductor device, a solder resist 103 having an opening is formed on a substrate 101. A Cu layer 105, a Ni layer 107 and an alloy layer 109 are laminated in turn inside this opening. The alloy layer 109 includes Cu, Ni and Sn as essential components. A Sn—Ag—Cu-based solder 111 is installed on the surface of the alloy layer 109.
FIG. 2 is a sectional view showing an example where the conventional semiconductor device is installed on a mount substrate. As shown in FIG. 2, the mount substrate has a solder resist 123 having an opening on a substrate 121. A Cu layer 125 is formed inside this opening. When the Sn—Ag—Cu-based solder 111 is mounted, an alloy layer 129 that includes Cu and Sn is formed between the Cu layer 125 and the Sn—Ag—Cu-based solder 111. The Cu layer 125 and the Sn—Ag—Cu-based solder 111 are bonded through this alloy layer 129.
On the other hand, Japanese Laid-Open Patent Application JP-P2001-156207A discloses a semiconductor device in which Sn—Zn-based solder balls are used as external connection terminals.
However, we have now discovered that the conventional technique has a room to be improved with regard to the following points.
Firstly, when the Sn—Ag—Cu-based solder described in the JP-A-Heisei, 5-50286 was used as the external connection terminals of the BGA, the impact resistance of the alloy layer 109, which included Cu, Ni and Sn and served as the solder bonding portion on the semiconductor device side, was lower than that of the conventional Sn—Pb eutectic solder. Thus, there was a case that the connection reliability was reduced.
Secondly, when the Sn—Zn-based solder described in the JP-P2001-156207A was used as the external connection terminals, the impact resistance of the alloy layer serving as the solder bonding portion on the semiconductor device side was improved. However, since the Sn—Zn-based solder was low in humidity resistance, there was a case that the connection reliability for a long term could not be maintained. Moreover, since the Zn included in the Sn—Zn-based solder was very high in reactivity with Cu, an alloy layer (Zn—Cu alloy layer) having a thick thickness is formed on the bonding surface. For this reason, the impact resistance of the alloy layer serving as the solder bonding portion on the mount substrate side was low, and there was a case that it was stripped when external force was applied. In this way, the mount substrate requires the limit even on the metal material of the portion bonded to the solder.
For this reason, a semiconductor device having solder balls as external connection terminals, which is superior in connection reliability to a mount substrate and superior even in a connection reliability for a long term, is desired.